1. Field of the Invention
The present invention generally relates to the field of optical communication and, more particularly to optical sampling.
2. Description of the Related Art
The monitoring of an optical channel quality is a challenging issue in all-optical networks, for which cost-effective, bit-rate and data-format flexible all-optical monitoring methods are needed to replace electronics-based methods. Given the limitations of the response speed of existing photodetectors, all-optical sampling is a promising enabling technology for optical networks with, for example, a data rate exceeding 40 Gb/s.
Sampling methods typically derive the time resolution needed to monitor the data waveforms from short temporal sampling gates. Such methods can be categorized by how the sampling gates are implemented. There are essentially three physical processes underpinning existing optical sampling techniques: nonlinear optical wave mixing between the short sampling pulses and data pulses, short optical gates from nonlinear optical interferometric switches, and linear optical homodyne detection of data using ultrashort optical pulses.
Current all-optical sampling methods are capable of measuring the eyes of optical data of a bit rate as high as 640-Gb/s, however, such methods are costly, complex, and relatively difficult to implement. The current methods are better suited as test and measurement tools in a laboratory environment rather than in actual networks. Specifically, these schemes typically require complex optical sources generating picosecond sampling pulses. The physical processes are inherently polarization and wavelength sensitive and can pose great difficulty in implementing all-optical sampling for actual networks, where the polarization of the data pulses can fluctuate rapidly, even from bit to bit, and different optical channels have different wavelengths in a WDM system. The typical remedies to combat these deficiencies, such as polarization-diversity schemes for the varying polarization of the data, tend to be complicated and costly. Additionally, a clock recovery circuit is necessary for synchronous sampling, where the repetition rate of the sampling gates is synchronized with the data, and it also increases the complexity and cost of the implementation.
Although electroabsorption modulators (EAMs) are attractive devices for generating short temporal gates, owing to the nonlinear optical-transmission as a function of the drive voltage, optical sampling using only one EAM poses several difficulties. These difficulties include, for example, the fact that the direct generation of picosecond electrical pulses of variable repetition rates less than about 100 MHz (the suitable rates for low-cost data acquisition and processing electronics) typically induces temporal jitters larger than several picoseconds, compromising the time resolution of the sampling. Other difficulties include the high cost of the wide bandwidth electronics required in the pulse generation process and pulse amplification to drive an EAM. Also, the extinction ratio of the temporal gates from an EAM tends to get worse when generating gates having shorter durations.